This invention relates to an improved sensor of the type having a ceramic element that undergoes a change in an electrical characteristic in response to a change in the partial pressure of oxygen in a mixture of gases to which the ceramic element is exposed. The ceramic element of the sensor may be either titania or zirconia under the current state of development, but other electrically responsive ceramics are known and may be used in the future. The preferred titania ceramic element is porous to provide a large surface area for effecting the transfer of oxygen from the titania to the gases to which the ceramic element is exposed and vice versa. The titania ceramic element has a porous or discontinuous coating of a precious metal charge transfer material. This material in the past has been platinum applied to the titania ceramic element by immersion in a solution containing platinum.
Sensors of the type having a zirconia ceramic element also utilize a porous platinum charge transfer material, but the zirconia ceramic is very dense and the platinum is applied to the zirconia surfaces by vapor deposition. The surface platinum to be exposed to engine exhaust gases is usually covered with a porous refractory material to aid in bonding and for the protection of the platinum.
Sensors of the type discussed above are particularly suited for use in detecting excursions, above and below stoichiometry, of the air-fuel ratio of the mixture of air and fuel supplied to an internal combustion engine. In accomplishing this detection, the sensor is positioned in the path of the exhaust gases emanating from the engine. As the mixture supplied to the engine changes from rich to lean, the exhaust gases change from a composition including very little oxygen to a composition containing an excess of oxygen. As the exhaust gases change from lean to rich, the reverse changes in composition occur. The sensors have an electrical characteristic that undergoes a step-function change as a result of the mixture excursions across the stoichiometric air-fuel ratio.
The titania ceramic material undergoes a change in its resistance as a function of the oxygen concentration gradient between the titania and the exhaust gases. The zirconia ceramic element undergoes a change in the EMF produced across its platinum change transfer electrodes as a function of the oxygen concentration differential on opposite sides of the zirconia material. With the zirconia sensor, a reference gas, usually air, is applied to one side of the zirconia and the exhaust gas composition is allowed to contact the other side of the zirconia. The use of a reference gas is unnecessary in connection with titania sensors, and the entire titania ceramic element is immersed in the exhaust gases.
The present invention is particularly directed to a titania sensor, but has possible application to zirconia sensors and others if problems peculiar to these sensors are eliminated or become less extreme as the art progresses.
The specific problem solved by the present invention is the loss of response to air-fuel mixture variations that occurs with the prior art titania sensor as a result of its use over a relatively short period of time. This loss of response occurs in the lower portion of the normal operating temperature range, which extends from about 300.degree. C. to about 900.degree. C. The failure of the sensor to operate at low temperatures due to loss of its low temperature response is a very serious problem because it means that the feedback fuel control system associated with the sensor for controlling the mixture ratios supplied to an internal combustion engine cannot be operated until the exhaust gases have heated the sensor sufficiently to maintain its temperature above that at which it is able to respond to air-fuel ratio variations. These may increase undesirable engine exhaust emissions and reduce fuel economy during engine warm-up conditions.